1. Field of the invention
This invention relates to certain new and useful improvements in apparatus for electroplating bulk parts.
2. Description of the prior art.
The electroplating of small metal or plastics parts on an industrial scale is today more and more widely carried out, for reasons of economy, in bulk within bell or drum-shaped containers. It is then possible for a bulk-mass of the small parts to be loaded into the containers, which being perforated can be partly dipped or wholly immersed in the appropriate electrolytes or other treatment liquids, and rotated therein. It has to be borne in mind that before the start of the electroplating process proper it is or may be necessary chemically to roughen the surface of bulk-parts made of plastics, generally by etching their surfaces, and thereafter to coat the roughened surfaces and render them electrically conductive by subsequent so-called electroless metal-deposition of a metallic layer. The conventional perforated bell-shaped or drum-shaped containers used for these purposes are generally known as tumbling barrels, and there are a variety of such conventional tumbling barrels which differ in their construction, dependent especially upon their intended use, above all according to whether they are meant for electroplating bulk parts made of metals or of plastics. The different forms of construction of the conventional tumbling barrels are thus influenced by the various properties which characterize the particular kind of bulk parts under treatment, above all by whether they are formed of metals or of plastics. There are no known tumbling barrels which are capable, with a standard constructional shape and unaltered method of operation, of properly satisfying the functional requirements which are both necessary and desirable for selectively bulk-electroplating parts made either of metals or of plastics in one and the same tumbling barrel.
In order clearly to see the deficiencies in the tumbling barrels of the prior art and to recognize the advantages of the solution provided according to the present invention it is convenient first to define those requirements which must be met for electroplating bulk parts made of plastics but to which an electroless-deposited metallic coating has been applied - because for the most part the same requirements are also of critical significance for the bulk-electroplating of parts made of metal.
When electroplating a bulk-mass of parts made of plastics and covered with an electrically-conducting metal layer, these usually float or are suspended in the electrolyte; but they will also sometimes sink to the bottom of the rotating container, whenever the average specific weight of the individual plastics part is greater or has grown greater than that of the electrolyte or other treating solution in the bath. The specific weights of the plastics themselves are mostly smaller than those of the electrolytes (around 1.040 g./c.c. for ABS-polymers as against 1.120 g./c.c. for a cyanide copper bath or 1.166 g./c.c. for a nickel bath) but the average specific weight of each individual part increases as the electrolessly-deposited metallic coating is built up on the surface of the part. The average specific weight of any part may be calculated as the quotient secured by the division of the sum made up of the weights of the plastics part and its continually-growing metal coating on the one hand by the total body volume of the part on the other hand.
With that in mind, the considerations affecting the problem may be enumerated as follows:
1. Contact Pressure PA0 2. Intermixing PA0 3. Relative Movement PA0 4. Fluid Boundary Layer PA0 5. Break up of the Electric Field PA0 a. Bi-polar Effects PA0 b. Unhomogeneous Partition of the Electrical Potential PA0 c. Chemical Corrosion PA0 d. Burn Patches PA0 A. batch Size Increase PA0 B. intensive Intermixing of Parts PA0 C. fixed Position of Batch PA0 D. increase in Specific Electrical Contact Pressure PA0 E. increased Electroplating Current
The force bringing two adjacent plastic parts in the bulk mass together in order to establish electrical contact between them as they are suspended in the electrolyte is extremely small due to the minimal difference between the average specific weight of the metal-coated plastic parts and the specific weight of the electrolyte.
In order to ensure the uniform appearance of all the parts in the load after electroplating it is necessary throughout the electroplating operation continuously to alter the position of each part relative to the load as a whole.
Equally, in order to achieve a uniform appearance in the electroplated product it is necessary for the parts to be continuously moving throughout the electroplating operation at differing speeds with respect to each other.
Whatever the nature of the plastics part itself, metal surfaces are generally not hydrophobic, and the metal-coated surface of the plastic part is therefore wetted by the electrolyte, which clings to the surface in the form of a layer which moves around with the part at the same speed.
The efficient electroplating of the rotating load of bulk parts takes place effectively at and in its peripheral zone. The electrical field is formed in the electrolyte between the anode (situated outside the immersed barrel) and the cathodically-polarized load (situated inside the barrel) but this field breaks down - in accordance with the Faraday cage-effect - at the edge of the polarized load, and is able to penetrate this only to an insignificant extent. Thus electroplating is confined practically to the edges of the load.
6. Electrolyte - exchange
A continuous turnover of the electrolyte at the cathode surface is advantageous, particularly as regards the deposition of polished platings, since this avoids impoverishment of the electrolyte adjacent the surface in the metal ions to be deposited and also removes the air bubbles present as well as the gas bubbles arising from the flow of electric current, which can stick to the surface of the bulk parts and cause local impediments to the deposition of the metal. The impoverishment of the electrolyte in the metal ions to be deposited and the formation of gas bubbles both take place principally where the current densities on the cathode are largest that is to say in the edge zones of the load, and thus at the peripheries of the rotating barrel. Thus electrolyte-exchange is particularly important at the peripheral circumference of the rotating tumbling barrel.
The considerations discussed at (1) to (4) above lead to the creation in actual practice of a hypothetically-avoidable film of electrolyte, varying in thickness, between the adjacent bulk parts. The existence of this separating film of electrolyte leads to certain undesirable consequences, as follows:
The electroplating current flows from the anode, constituting the positive pole of the system and situated outside the electroplating tumbling barrel, through the electrolyte in the electroplating bath and through the load (or more strictly through the metal "skin" on the plastics parts) right through to the cathodic contact elements, constituting the negative pole of the system, which are situated in the rotatable tumbling barrel. However when the individual parts are separated ("insulated") from one another by a film of electrolyte then on the individual plastics part the metal coating retains the function of an electrical interconnector, but the part acts as a bi-pole within the load. One area on its metallic surface displays a cathodic potential and another area, electrically-speaking diametrically opposite thereto, displays an anodic potential. Thus local galvanic cells are set up between adjacent plastics parts, oriented in the direction in which the electroplating current flow. Bipolar effects unfortunately cause at least partial electrolytic redissolution of the metal coating (no matter whether deposited electrolessly or galvanically) in those areas of the surface of the parts which (temporarily in the course of the rotational movement) are at anodic potential. Consequently such bi-polar effects lead to the reversal of the desired proper electroplating effects. Moreover, these bi-polar effects also lead to the electrolytic formation of metal oxides on the metallic coating layer during its local, anodic polarization, thereby causing rough and matt surfaces which are unsuitable for purposes of decoration.
The electroplating current does not distribute itself uniformly, and anyway its pattern of flow is not easily visible and cannot be controlled. The consequence is a lack of uniformity in the appearance of the bulk parts and - within the limits of the decreasing potential differences of the individual bulk parts with respect to the anode system -also a substantial decrease in the speed of electrolytic deposition, thus in the electroplating performance.
It is impossible completely to eliminate the chemical attack of the electrolyte on the metal coating upon the plastics part in the bath, owing to the breakdown of the electrical field at the edge of the load, and this corrosive effect of the solution is significantly promoted by the existence and extent of the separating films of electrolyte. The resultant corrosion leads to the chemical redissolution of the deposited metal; and almost always also leads to the chemical oxidation of the surface of the deposited metal.
The phenomenon of so-called burn patches occurs either upon the conductive electrolessly-deposited coating or upon the subsequent galvanically-applied electroplating upon the plastic parts. These burn patches appear not as points but are spread over areas which generally cover a considerable part of the surface of the part. Their occurrence can be explained as follows. Galvanic baths mostly are worse electrical conductors than metals; their respective conductive capacities for electricity differ by a ratio whose order of magnitude is about 1:10.sup.5. As indicated at (a) above, the setting up of local galvanic cells between the adjacent bulk parts is to be expected; and the passage of current is then concentrated (in the region involved in the transmission of current from one part to the other) first of all on a gap which spatially is almost point-shaped, comprising the electrically-opposed, polarized areas of the metal coatings on two adjacent bulk parts and the electrolyte film lying between them. The Joule heat evolved corresponds to the product of the electric current i squared and of the electrical resistance R of the system. The resistance R is minimal if metal bridges are in existence, that is if direct metal contacts have been established between one part of the load and another; but on the other hand the resistance R suddenly jumps if the current conduction has to take place across a bad electrical conductor, to be specific in this case across an intervening film of electrolyte. It ought also to be borne in mind that on the anodic side of the system (consisting of two adjacent bulk parts) redissolution of the metal layer as well as oxidation of the surface is liable to take place; both of these effects lead to impairment of conductivity, for metal oxides are well known to be generally bad electrical conductors. Thus, all these previously-described effects may be superimposed to bring about a localized high total resistance R, and thus an excessive, local generation of heat. This results in dark violet-coloured burn patches and circular spots on the surface of the bulk parts, which may also be partially or completely stripped of their metal covering layer by these electrolytic and thermal reactions. Dark burn patches and de-metallized spots on the bulk parts are not rare occurrences; on the contrary it is a not uncommon experience for anyone with knowledge of this field to encounter a completely failed load, in which virtually all the bulk parts - piece after piece-prove to be unuseable.
The known tumbling barrels include those of prismatic or cylindrical shape arranged to rotate around their horizontal axis of symmetry, which dip only a relatively small part of their volume into the electrolyte and possess an opening centrally-located in one of the two end walls mounted perpendicular to the axis of rotation, this opening serving for loading or unloading the batch of metal bulk parts. Since this opening remains essentially free during the electroplating it need not be covered with a lid. Tumbling barrels of the kind just described are however unsuited for electroplating metallized plastics parts, as these float or are suspended in the electrolytic solution in the bath.
Alternatively, tumbling barrels of prismatic shape with a hexagonal cross-section have been marketed, which similarly rotate about their horizontal axis of symmetry but have an opening for loading or unloading the batch on the peripheral circumference of the casing, which of course must be closed by means of a lid during electroplating. These barrels have a length of about 550 mm. and an average casing diameter of about 200 mm. The electroplating current is carried to the load via two insulated cables which are individually introduced within the barrel through the two axial bearings which support the barrel at or adjacent its end walls, and these cables terminate in smooth, metallic, cylindrical contact elements, about 200 mm. long and about 12 mm. in diameter. This type of tumbling barrel can be used without constructional alteration for electroplating bulk parts no matter whether made of metal or made of plastics - though the conditions of application have to be altered appropriately. When electroplating bulk parts made of metal such tumbling barrels can be immersed completely or nearly completely in the electrolyte, but when electroplating bulk parts made of plastics, then according to Muller, G., "Galvanisieren von Kunststoffen", published by E. G. Leuze Verlag, Saulgau, (1966), pgs. 104-105, one must
".....allow these commercially-available electroplating barrels to be dipped only partially, that is up to one third to 50%. In this way the goods are compelled to pack together at the bottom of the barrel and a more constant contact is established. Since however like this only a very small volume of liquid is effective, the current densities which can be employed are very small, resulting in long treatment times. The constant rising of the contacts out of the liquid leads to the passivation of the electrical contacts." It is in fact standard industrial practice to load tumbling barrels with a batch of parts - irrespective of whether those parts are made of metal or plastics - which fills only from about one third up to at most one half of the space within the barrel. Experience has shown that when filled with larger quantities the intermixing of the load during electroplating becomes unsatisfactory, resulting in unevenly-electroplated bulk parts and very long electroplating times.
A tumbling barrel has been proposed in U.S. Pat. No. 3,330,753 which consists principally of two end walls vertical to the axis of rotation, an outer casing fixed between these end walls and surrounding an inner, concentrically-arranged coaxial cylinder, and several rod-shaped cathode contacts extending between the end walls in the annular space between the outer barrel casing and the inner cylinder. It is there suggested that this annular space within the barrel should be filled as nearly full as possible so long as the bulk parts can still move; the purpose of filling the barrel nearly full is to restrict the room for movement of the plastics parts as much as possible in order to force them to the opposite electrical contact.
However, this has its disadvantages. As explained at (5) above, the electrical field breaks up at the peripheral circumference of the large-volume load. Since the majority of the bulk parts lie within the conglomerate mass they consequently are effectively shut off from the galvanic plating process, and as explained at (a), (c) and (d) they run increased risk of bi-polar effects, of chemical corrosion by the electrolyte, and of the formation of burn patches. Moreover, it is also easy to see the irregular distribution of the electrical potential field in the load, explained at (b) above, when such a tumbling barrel is put into use. Thus for example when the bulk parts are relatively small, by opening the barrel lid and reaching into the load one can clearly see that the bulk parts near the peripheral circumference have already been plated with electrolytically-deposited metal at a time when parts located well inside of the batch have been electroplated only very slightly or even not at all. This means at best that extremely long electroplating times are needed, while the platings produced on the bulk parts in the load are liable to look uneven, varying from matt to brightly-shining.
Tumbling barrels have been proposed in German Pat. Nos. 277,128 and 281,032 whose length is smaller than their diameter, and whose end walls normal to the axis of rotation are perforated. The barrel according to German Pat. No. 277,128 has a peripheral casing of metal, which serves as the cathode contact element for the batch loaded into the barrel. The direct current needed for electroplating is led to the casing via a flexible metal band, which extends more than half way around the electrically-conductive barrel casing and at the same time holds the barrel in its operative rotating position. The barrel according to German Pat. No. 281,032 makes cathodic contact with the batch via a metal hoop fastened on the inside of the peripheral barrel casing. The direct current needed for electroplating is led to the contact hoop along several radially disposed spokes, which radiate from a metal driving wheel (for rotating the barrel) secured concentrically with the barrel.
However the two different sorts of tumbling barrels proposed in German Pat. Nos. 277,128 and 281,032 have not found favour in industrial practice, for several reasons. The current-conducting elements of the barrels (the flexible band, the spokes and the driving wheel) all quickly become covered with electrolytically-deposited metal, since the electrical resistance between them and the anode system tends to be much smaller than that between the cathodically-polarized load enclosed within the barrel and the same anode system. As a direct consequence of this unwanted metal deposition the barrel in each case may become incapable of functioning mechanically within quite a short time.
A further important reason for the practical failure of the tumbling barrels just described above lies in the deviation of the electroplating current away from the load of bulk parts. The tendency is for most of the electroplating current to flow to the barrel elements at cathodic potential (flexible band, spokes and driving wheel) so that only a fraction of the electroplating current flows to the barrel and reaches the load, because the cathodic contact elements (the contact casing and the metal hoop) on the periphery of the barrels not only display a higher potential difference but also have a more favourable position with respect to the anode system than the load. As explained at (5) above, the electroplating process takes place at the edge of the load; and from this it is self-evident that the disposition of the contact-casing and -hoop on the outside of these conventional barrels causes them to draw a considerable part of the remaining galvanic current to themselves, to the detriment of the load. For the reasons explained at (a), (b), (c) and (d) above, the resultant galvanic platings will tend to be rough and matt, possibly dark-violet and of generally differing aspect, so that one gets a possibly useless product even after very long electroplating times.
Yet another disadvantage of these same conventional tumbling barrels derives from the fact that, as previously indicated they are filled with the load only to about a third of their volume. There is consequently only a small specific contact pressure between the individual bulk parts made of plastics and between these bulk parts and the cathodic contact elements. This inadequate contact pressure is a direct result of filling not more than a third of the volume of the barrel, and it leads to bi-polar effects and an irregular electrical potential field in the load, thus causing burn patches on the bulk parts, chemical re-dissolution of the deposited metal coatings and hence a regular percentage of supposedly electroplated parts in the load which have to be rejected as useless.
The tumbling barrel according to German Pat. No. 277,128 is particularly unsuitable for electroplating plastics parts, since one of its features is the division of its volume into self-contained compartments.
As explained at (5) above, the electrical field breaks up, as is well known at the peripheral circumference of the load - and the galvanic reaction thus takes place practically only at the edge of the conglomerate of bulk parts. If by F [expressed in dm.sup.2 ] one denotes the external surface (the surface of the peripheral circumference) of the load, and by V [expressed in dm.sup.3 ] one denotes its volume, then the quotient F/V [dm.sup.2 /dm.sup.3 ] defines the specific average value and thus the proportion of the electroplating current I per unit of space of the volume of the load. If the value of this specific quotient F/V is relatively high, then the speed with which electroplating takes place in the barrel is likewise proportionally high. All the conventional tumbling barrels are however characterized by low specific average values F/V of the electroplating current I. This common disadvantage is of extreme importance from a chemical engineering standpoint; it leads to very long electroplating times, and therefore small electroplating throughputs per barrel. This applies just as much when the load consists of metal parts as when it consists of plastic parts.